CS6250 EXAM 1 COMPUTER NETWORKS(2024-
2025) QUESTIONS AND ANSWERS
● Explain Additive Increase/Multiplicative Decrease (AIMD)
in the context of TCP. - <,Answers>>>TCP decreases the
window when the level of congestion goes by halving the
window size, and it increases the window when the level of
congestion goes down by adding to the window size. This
causes convergence to the optimal bandwidth by quickly
cutting use in times of congestion while slowly increasing
utilization when the congestion clears.
The idea behind additive increase is to increase the window
by one packet every RTT (Round Trip Time).
Once TCP Reno detects congestion, it reduces the rate at
which the sender transmits. So, when the TCP sender detects
that a timeout occurred, then it sets the CongestionWindow
(cwnd) to half of its previous value.
● Describe the relationships between ISPs, IXPs, and CDNs. -
<,Answers>>>The Internet is a complex ecosystem. It is built
with a network of networks. These networks include (ISPs)
Internet Service Providers, (IXPs) Internet Exchange Points,
(CDNs) Content Delivery Networks. In 2019, there were apx.
500 IXPs around the world.
large scale Tier-1 ISPs that operate at a global scale, and
essentially they form the "backbone" network over which
smaller networks can connect
, CS6250 EXAM 1 COMPUTER NETWORKS(2024-
2025) QUESTIONS AND ANSWERS
IXPs are interconnection infrastructures, which provide the
physical infrastructure, where multiple networks (eg ISPs and
CDNs) can interconnect and exchange traffic locally.
CDNs are networks that are created by content providers with
the goal of having greater control of how the content is
delivered to the end-users, and also to reduce connectivity
costs. Some example CDNs include Google and Netflix.
These networks have multiple data centers - and each one of
them may be housing hundreds of servers - that are distributed
across the world.
● What is an AS? - <,Answers>>>AS stands for autonomous
systems. ISPs, IXPs and CDNs can all operate as an AS. An
AS is a group of routers including the links among them that
operate under the same administrative authority.
● What kind of relationship does AS have with other parties?
- <,Answers>>>An AS is independent of other ASes. They set
their own policies, make its own traffic engineering decisions
and interconnection strategies and also determine how the
traffic leaves and enters the network.
● What is BGP? - <,Answers>>>BGP stands for Border
Gateway Protocol. The border routers of ASes use BGP to
exchange routing information with each other.
, CS6250 EXAM 1 COMPUTER NETWORKS(2024-
2025) QUESTIONS AND ANSWERS
● How does an AS determine what rules to import/export? -
<,Answers>>>AS business relationships drive an AS' routing
policies and influence which routes an AS needs to import or
export.
There are 3 selective transit routes: Transit customer routes,
Transit Provider Routes, Peer Routes. The main rule is
financial incentive. Transit customer routes > peer routes >
transit provider routes.
Similarly as exporting, ASes are selective about which routes
to import based, primarily, on which neighboring AS
advertises them and what type of business relationship is
established. An AS receives route advertisements from its
customers, providers and peers.
When an AS receives multiple route advertisements towards
the same destination, from multiple ASes, then it needs to
rank the routes before selecting which one to import. The
routes that are preferred first are the customer routes, then the
peer routes and finally the provider routes. The reasoning
behind this ranking is that an AS...
1. wants to ensure that routes towards its customers do not
traverse other ASes unnecessarily generating costs,
2. uses routes learned from peers since these are usually "free"
(under the peering agreement),
3. and finally resorts to import routes learned from providers
as these will add to costs.
, CS6250 EXAM 1 COMPUTER NETWORKS(2024-
2025) QUESTIONS AND ANSWERS
● What are / were the original design goals of BGP? What
was considered later? - <,Answers>>>The original design
goals of BGP were:
1. Scalability
manage the complications of this growth, while achieving
convergence in reasonable timescales and providing loop-free
paths.
2. Express routing policies
BGP has defined route attributes that allow ASes to
implement policies (which routes to import and export),
through route filtering and route ranking
3. Allowing cooperation among ASes
individual AS can still make local decisions (which routes to
import and export) while keeping these decisions confidential
from other ASes.
Security was considered later as the complexity and size of
the Internet has been increasing. Solutions have not been
widely deployed or adopted due to difficulty to transition to
new protocols and lack of incentives.
● What are the basics of BGP? - <,Answers>>>BGP session.
A pair of routers, known as BGP peers, exchange routing
information over a semi-permanent TCP port connection
called a BGP session. To begin a BGP session a router will
2025) QUESTIONS AND ANSWERS
● Explain Additive Increase/Multiplicative Decrease (AIMD)
in the context of TCP. - <,Answers>>>TCP decreases the
window when the level of congestion goes by halving the
window size, and it increases the window when the level of
congestion goes down by adding to the window size. This
causes convergence to the optimal bandwidth by quickly
cutting use in times of congestion while slowly increasing
utilization when the congestion clears.
The idea behind additive increase is to increase the window
by one packet every RTT (Round Trip Time).
Once TCP Reno detects congestion, it reduces the rate at
which the sender transmits. So, when the TCP sender detects
that a timeout occurred, then it sets the CongestionWindow
(cwnd) to half of its previous value.
● Describe the relationships between ISPs, IXPs, and CDNs. -
<,Answers>>>The Internet is a complex ecosystem. It is built
with a network of networks. These networks include (ISPs)
Internet Service Providers, (IXPs) Internet Exchange Points,
(CDNs) Content Delivery Networks. In 2019, there were apx.
500 IXPs around the world.
large scale Tier-1 ISPs that operate at a global scale, and
essentially they form the "backbone" network over which
smaller networks can connect
, CS6250 EXAM 1 COMPUTER NETWORKS(2024-
2025) QUESTIONS AND ANSWERS
IXPs are interconnection infrastructures, which provide the
physical infrastructure, where multiple networks (eg ISPs and
CDNs) can interconnect and exchange traffic locally.
CDNs are networks that are created by content providers with
the goal of having greater control of how the content is
delivered to the end-users, and also to reduce connectivity
costs. Some example CDNs include Google and Netflix.
These networks have multiple data centers - and each one of
them may be housing hundreds of servers - that are distributed
across the world.
● What is an AS? - <,Answers>>>AS stands for autonomous
systems. ISPs, IXPs and CDNs can all operate as an AS. An
AS is a group of routers including the links among them that
operate under the same administrative authority.
● What kind of relationship does AS have with other parties?
- <,Answers>>>An AS is independent of other ASes. They set
their own policies, make its own traffic engineering decisions
and interconnection strategies and also determine how the
traffic leaves and enters the network.
● What is BGP? - <,Answers>>>BGP stands for Border
Gateway Protocol. The border routers of ASes use BGP to
exchange routing information with each other.
, CS6250 EXAM 1 COMPUTER NETWORKS(2024-
2025) QUESTIONS AND ANSWERS
● How does an AS determine what rules to import/export? -
<,Answers>>>AS business relationships drive an AS' routing
policies and influence which routes an AS needs to import or
export.
There are 3 selective transit routes: Transit customer routes,
Transit Provider Routes, Peer Routes. The main rule is
financial incentive. Transit customer routes > peer routes >
transit provider routes.
Similarly as exporting, ASes are selective about which routes
to import based, primarily, on which neighboring AS
advertises them and what type of business relationship is
established. An AS receives route advertisements from its
customers, providers and peers.
When an AS receives multiple route advertisements towards
the same destination, from multiple ASes, then it needs to
rank the routes before selecting which one to import. The
routes that are preferred first are the customer routes, then the
peer routes and finally the provider routes. The reasoning
behind this ranking is that an AS...
1. wants to ensure that routes towards its customers do not
traverse other ASes unnecessarily generating costs,
2. uses routes learned from peers since these are usually "free"
(under the peering agreement),
3. and finally resorts to import routes learned from providers
as these will add to costs.
, CS6250 EXAM 1 COMPUTER NETWORKS(2024-
2025) QUESTIONS AND ANSWERS
● What are / were the original design goals of BGP? What
was considered later? - <,Answers>>>The original design
goals of BGP were:
1. Scalability
manage the complications of this growth, while achieving
convergence in reasonable timescales and providing loop-free
paths.
2. Express routing policies
BGP has defined route attributes that allow ASes to
implement policies (which routes to import and export),
through route filtering and route ranking
3. Allowing cooperation among ASes
individual AS can still make local decisions (which routes to
import and export) while keeping these decisions confidential
from other ASes.
Security was considered later as the complexity and size of
the Internet has been increasing. Solutions have not been
widely deployed or adopted due to difficulty to transition to
new protocols and lack of incentives.
● What are the basics of BGP? - <,Answers>>>BGP session.
A pair of routers, known as BGP peers, exchange routing
information over a semi-permanent TCP port connection
called a BGP session. To begin a BGP session a router will
